The mobility of alkali ions in gases II. The attachment of inert gas atoms to alkali ions

During the course of some measurements of the mobility of lithium ions in xenon, it was observed that with small fields an ion of low mobility appeared in addition to the normal lithium ion. The first interpretation to suggest itself was that the molecules of a polar impurity in the gas were responsible for the formation of a clustered ion. But despite repeated attempts at purification this ion did not disappear, and since it could be partially suppressed by reducing the pressure or increasing the fields, it was not studied in detail at the time. However, during a later investigation by David and one of us (K. H., unpublished) with lithium ions in helium at liquid hydrogen temperature, conclusive evidence was obtained of a similar heavy ion at low fields. In this case there was no possibility of the ion picking up polar molecules of impurity since these would all be frozen out. The helium used was also claimed to be spectroscopically pure, so that undoubtedly the heavy ion arose from the attachment of helium atoms to the lithium ion. The polarizability of an inert gas atom increases with its atomic weight, so that on general grounds one would expect that the temperature range over which clusters of these atoms with a lithium ion could be detected would also increase with the atomic weight of the gas atom. The fact that clustered ions were observed in xenon at room temperature is therefore understandable, if they consisted of one or more xenon atoms attached to a lithium ion. Subsequent experiments were therefore undertaken in the other gases and the results are recorded and discussed below.